It's look like that either Me.ScheduledNumber OR Me.CourseMInDel has a Null Value " " in your your field.
if so, then nz() function should solve this:
Private Sub Frame76_AfterUpdate()
If (Me.chkFinal.Value = -1) And nz((Me.ScheduledNumber < Me.CourseMInDel),0) Then
MsgBox "There are less...